Capacitance-type material level indicator

ABSTRACT

A system and probe for indicating the level of material in a vessel as a function of material capacitance comprising a resonant circuit including a capacitance probe adapted to be disposed in a vessel so as to be responsive to variations in capacitance as a function of material level. An rf oscillator has an output coupled to the resonant circuit and to a phase detector for detecting variations in phase angle as a function of probe capacitance. Level detection circuitry is responsive to an output of the phase detector and to a reference signal indicative of a predetermined level of material for indicating material level as a function of a difference between capacitance at the probe and the reference signal. In the preferred embodiments of the invention disclosed, an automatic calibration circuit adjusts the resonance characteristics of the parallel resonant circuit of the reference signal indicative of a predetermined reference material level.

The present invention is directed to systems for indicating level ofmaterial in a storage vessel or the like, and more particularly to animproved system of the described type for indicating material level as afunction of material capacitance. The present invention also relates toa capacitance material sensing probe and to a method for manufacturethereof.

OBJECTS AND SUMMARY OF THE INVENTION

A general object of the present invention is to provide a system forindicating the level of material in a storage vessel or the like whichis inexpensive in manufacture and reliable in operation over asubstantial operating lifetime and in a variety of operatingenvironments.

Another and more specific object of the present invention is to providea material level indicating system of the described type which may bereadily calibrated in the field by relatively unskilled personnel for avariety of applications and environments. A related object of theinvention is to provide such a system to include facility for rapid andautomatic recalibration in the field by an unskilled operator.

A further object of the invention is to provide a capacitance-typematerial level indicating system with reduced sensitivity to the effectsof material coating on the capacitance probe and/or to the effects ofconductivity or variation in conductivity of the sensed material.

Yet another object of the present invention is to provide an improvedcapacitance sensing probe for application in material level indicatingsystems, and an inexpensive method for manufacture of such a probe.

Briefly stated, the present invention contemplates a system forindicating the level of material in a vessel as a function of materialcapacitance a resonant circuit including a capacitance probe adapted tobe disposed in a vessel so as to be responsive to variations incapacitance as a function of material level in the vessel, an oscillatorhaving an output coupled to the resonant circuit including thecapacitance probe, a phase detector responsive to variations in phaseangle at the oscillator output as a function of probe capacitance, acalibration circuit for identifying a reference capacitance indicativeof a predetermined level of material in the vessel, and an outputcircuit responsive to the phase detector and calibration circuit forindicating material level in the vessel as a function of a differencebetween capacitance at the probe and the reference capacitance. Thecalibration circuit includes a comparator having a first inputresponsive to the phase detector and a second input indicative of thereference capacitance. Operating characteristics of the system arevaried during a calibration operation to obtain a predeterminedcomparison at the comparator, preferably substantially at resonance ofthe resonance circuit. Most preferably, the calibration circuit operatesautomatically upon initiation of a calibration operation to vary systemcharacteristics, such as, the resonance characteristics of the resonantcircuit on the reference input to the comparator, to obtain the desiredpredetermined operation substantially at resonance.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with additional objects, features and advantagesthereof, will be best understood from the following description, theappended claims and the accompanying drawings in which:

FIG. 1 is a functional block diagram of a presently preferred embodimentof a capacitance-type material level indicating system in accordancewith the invention;

FIGS. 2 and 3 are electrical schematic diagrams of respective portionsof the system illustrated in functional form in FIG. 1;

FIGS. 4 and 5 are electrical schematic diagrams of respectivealternative embodiments to the preferred embodiment as illustrated inFIG. 2; and

FIG. 6 is a partially sectional fragmentary elevational view of acapacitance sensing probe in accordance with a presently preferredembodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a presently preferred embodiment of the materiallevel indicating system of the invention as comprising an rf oscillator10 which provides a periodic signal at a first output to a phase shift(90°) amplifier 12. The sinusoidal output of amplifier 12 is connectedto an adjustable parallel LC resonant circuit 14. Resonant circuit 14 isconnected to the probe conductor 18 of a probe assembly 20 (FIGS. 1 and6) mounted in the side wall of a storage vessel 22. The output ofamplifier 12 is also connected through a unity gain amplifier 24 havinglow output impedance to the guard shield 26 of probe assembly 20. Thewall of vessel 22, which may be a storage bin for solid materials or aliquid storage tank, is connected to ground. As is well-known in theart, the capacitance between probe conductor 18 and the grounded wall ofvessel 22 varies with the level of the material 28 stored therein andwith material dielectric constant. This variation in the capacitance issensed by the remainder of the system electronics to be described andprovides the desired indication of material level. Guard shield 26,which is energized by amplifier 24 at substantially the same voltage andphase as probe conductor 18, functions to prevent leakage of probeenergy through material coated onto the probe surface, and thus todirect probe radiation outwardly into the vessel volume so as to be moreclosely responsive to the level of material stored therein.

The sinusoidal output of amplifier 12 is fed through a zero crossingdetector 30 to one input of a phase detector 32. Phase detector 32receives a square wave second input from a second output of oscillator10, 180° out of phase with the oscillator output directed to amplifier12. A first output of phase detector 32, which is a d.c. signal at alevel proportional to the phase relationship between the respectiveinputs, and thus responsive to variations in phase angle of theoscillator probe drive output due to changes in probe capacitance, isfed to an automatic calibration circuit 34. A second output of phasedetector 32, which is also a d.c. signal indicative of input phaserelationship, is directed to one input of a threshold detector 36. Theoutputs of phase detector 32 are identical but effectively isolated fromeach other for reasons that will become apparent. Automatic calibrationcircuit 34 provides a control input to adjustable LC resonant circuit 14which receives a second input for adjustment purposes from oscillator10. Calibration circuit 34 also provides a reference input to thresholddetector 36. The output of threshold detector 36 is fed through materiallevel indicating circuitry 38 to external circuitry for controllingand/or indicating vessel material level as desired.

In general, automatic calibration circuitry 34 functions to adjust theresonance characteristics of resonant circuit 14 during a calibrationmode of operation initiated by an operator push-button 40 connectedthereto so as to establish, in effect, a reference capacitance levelindicative of a preselected material condition in vessel 22 which existsduring the automatic calibration mode. Preferably, the level of materialin vessel 22 is first raised (by means not shown) to the level of probeassembly 20 and then lowered so as to be spaced from the probe assembly.If material 28 is of a type which coats the probe assembly, such coatingwill remain on the probe and be taken into consideration during theensuing calibration operation. With the material level lowered, anoperator may push button 40 to initiate the automatic calibration modeof operation. The resonance characteristics of circuit 14 are thenautomatically varied or adjusted by calibration circuit 34 until theoutput of phase detector 32 indicates that the return signal from theparallel combination of resonant circuit 14 and capacitance probe 20bears a preselected phase relationship to the oscillator reference inputto phase detector 32, which phase relationship thus corresponds to aneffective reference capacitance level at calibration circuit 34indicative of a low material level.

Thereafter, during the normal operating mode, the output of phasedetector 32 is compared in threshold detector 36 to a reference inputfrom calibration circuit 34 indicative of the reference capacitancelevel, and threshold detector 34 provides an output to material levelindicating circuitry 38 when the sensed material capacitance exceeds thereference capacitance level by a predetermined amount which is selectedas a function of material dielectric constant. If probe 20 is placed inthe upper portion of vessel 22 as shown in FIG. 1, such proximity wouldnormally indicate a full tank condition. If, on the other hand, probe 20is disposed in the lower portion of tank 22, material would normally bein proximity to the probe assembly, and indeed would normally cover theprobe assembly, so that absence of such proximity would indicate anempty tank condition.

FIG. 2 illustrates a presently preferred embodiment of automaticcalibration circuitry 34 and adjustable LC resonant circuit 14. Resonantcircuit 14 includes a fixed capacitor 42 and an inductance 44 connectedin parallel with probe conductor 18 across the output of amplifier 12,i.e. between the amplifier output and ground. Inductance 44 comprises aplurality of inductor coils or windings having a number of connectiontaps at electrically spaced positions among the inductor coil turns. Aplurality of fixed capacitors 46a-46f are each electrically connected inseries with a respective controlled electronic switch 48a-48f between acorresponding connection tap on inductance coil 44 and electricalground. Switches 48a-48f may comprise any suitable electronic switchesand are normally open in the absence of a control input. A digitalcounter 50 receives a count input from oscillator 10 and provides aplurality of parallel digital outputs each indicative of a correspondingbit of the count stored in counter 50. Each data bit output of counter50 is connected to control a corresponding electronic switch 48a-48f forselectively connecting or disconnecting the corresponding capacitor46a-46f in resonant circuit 14 as a function of the state of the counteroutput bit.

Most preferably, and in accordance with an important feature of thepreferred embodiment illustrated in the invention of FIG. 2, thecapacitance values of capacitors 46a-46f and the number of coil turnsseparating the connection taps of inductance 44 are selected such thatthe effective capacitance added to the parallel LC resonant circuit 14by each capacitor 46a-46f corresponds to the numerical significance ofthe corresponding counter output. That is, assuming that counter 50 is abinary counter with outputs connected to switches 48a-48f in reverseorder of significance, the values of capacitors 46e, 46f and the numberof turns at inductance 44 therebetween are selected such that theeffective capacitance connected in parallel with fixed capacitor 42 andprobe 20 is twice as much when switch 48e only is closed as when switch48f is only closed. Likewise, the effective capacitance added by switch48a and capacitor 46a is thirty-two times the effective value ofcapacitor 46f and switch 48f. It will be appreciated that inductance 44functions as an autotransformer so as to establish the effectivecapacitance of each capacitor 46 as a function of the correspondingconnection point among the inductance coils. It will also be appreciatedthat the number of inductance connection taps may be less than thenumber of capacitors 46a-46f, with two or more capacitors connected toone tap. The values of capacitors connected to a common tap shoulddiffer by multiples of approximately two in correspondence with thesignificance of the control bits from counter 50.

Automatic calibration circuit 34 illustrated in FIG. 2 includes a oneshot 52 which receives an input from operator push-button 40 andprovides an output to the reset input of counter 50 in resonant circuit14 to initiate the automatic calibration mode of operation. Adifferential comparator 54 has an inverting input connected to theoutput of phase detector 32 and a non-inverting input connected to thewiper of a variable resistor 56. Resistor 56 is connected across asource d.c. potential. The output of comparator 54 is connected to theenabling input of counter 50 in resonant circuit 14. The output ofcomparator 54 is also connected through a resistor 57 to the base of anNPN transistor 58 which functions as an electronic switch having primarycollector and emitter electrodes connected in series with an LED 60, aresistor 61 and operator switch 40 across a source of d.c. potential.The non-inverting input of comparator 54 is also connected through anadjustable resistor 62 to threshold detector 36 (FIGS. 1 and 3).

Depression of switch 40 by an operator initiates the automaticcalibration procedure by clearing or resetting counter 50. Allcapacitors 46 are disconnected from resonant circuit 14. With materialcoated on the probe, circuit operation is substantially removed fromresonance on the "inductive" side, and the output from phase detector 32to comparator 54 is high. Differential comparator 54 thus provides a lowoutput to the enabling input of counter 50 and to the base of transistor58, so that transistor 58 is biased for non-conduction and deenergizesLED 60. With counter 50 so reset and enabled, the pulsed counter inputfrom oscillator 10 advances the count in counter 50, and therebysequentially and selectively connects the various capacitors 46a-46finto the parallel LC resonant circuit as controlled by switches 48a-48f.As previously indicated, the effective capacitance added by connectionof each capacitor is directly related and proportional to the numericalsignificance of the corresponding bit in counter 50.

As capacitors 46 are added in parallel connection with inductance 44,capacitor 42 and probe 20, and as the parallel combination approachesresonance at the frequency of oscillator 10, the output of phasedetector 32 decreases toward the reference level determined by thesetting of variable resistor 56 at the non-inverting input ofdifferential comparator 54. Resistor 56 is preferably factory set tocorrespond with a resonance condition at circuit 14 for a low-level or"empty-vessel" nominal capacitance with no coating on probe 20 and allcapacitors 46a-46f in circuit. The empty-tank capacitance at probe 20may be 15 picofarads, for example. When the output of phase detector 32reaches this reference capacitance level input to comparator 54, whichis preferably at substantially the resonance condition of the LCresonant circuit, the output of differential amplifier 54 switches to ahigh or one logic stage. Further operation of counter 50 is inhibitedand LED 60 is illuminated through transistor 58 so as to indicate to anoperator that the calibration operation has been completed. The operatormay then release switch 40. Thus, the resonance circuit is designed tobe at resonance with all capacitors 46a-46f in circuit and the probeuncoated. The automatic calibration operation functions to delete one ormore capacitors 46a-46f from the parallel resonance circuit tocompensate for the coating on the probe, cable capacitance, tankgeometry, parasitic capacitance, and variations in probe insertionlength and circuit operating characteristics.

All of the circuitry hereinabove (and hereinafter) described receiveinput power from a suitable power supply (not shown) energized by autility power source. Preferably, adjustable LC resonant circuit 14further includes a battery 64 connected by the blocking diodes 66, 68 inparallel with the power supply d.c. voltage to the power input terminalof counter 50 so as to maintain the calibration count therein in theevent of power failure.

Referring now to FIG. 3, threshold detector 36 includes a differentialcomparator 70 having an inverting input connected to the second outputof phase detector 32 (FIG. 1) and a non-inverting input connectedthrough adjustable resistor 62 (FIG. 2) to reference-indicatingadjustable resistor 56. A pair of capacitors 72, 74 are connectedthrough corresponding jumpers 76, 78 between the inverting input ofcomparator 70 and ground. A third capacitor 75 is also connected betweenthe inverting input of comparator 70 and ground. Capacitors 72, 74, 75and jumpers 76, 78 provide a factory-selectable or field-selectableadjustable delay in operation of threshold detector 36 so that atransient condition will not result in an erroneous indication of changeof material level. The phase detector outputs are isolated as previouslydescribed so that delay capacitors 72, 74, 75 will not affect operationin the calibration mode. A pair of resistors 80, 82 are connected bycorresponding jumpers 84, 86 between the non-inverting input ofcomparator 70 and ground. A third resistor 87 is directly connectedbetween the non-inverting comparator input and ground. Resistors 80, 82,87 and jumpers 84, 86 cooperate with resistor 62 (FIG. 2) to providefactory or field selectable adjustment of the capacitance differentialsensed by threshold detector 36 between the reference set by resistor 56(FIG. 2) and the material-proximate material level which probe 20 isintended to indicate.

More specifically, with probe assembly 20 mounted in the upper portionof vessel 22 as illustrated in FIG. 1, the difference between thecapacitance level at probe 20 corresponding to the reference level ofresistor 56 with the material at low level and a full-tank capacitanceis the capacitance increase caused by rise in material level intoproximity with the capacitance probe. Resistors 80, 82, 87 and jumpers80, 84 effectively select the capacitance differential to be sensedbetween low and high material level conditions. For a material of lowdielectric constant such as cement, jumpers 84, 86 are removed and athreshold level corresponding to a capacitance differential of fourpicofarads, for example, is established by resistors 62, 87. For amaterial of medium dielectric constant such as acetone, jumper 84 may beadded so that resistors 80, 87 in parallel establish a highercapacitance differential of eight picofarads, for example, correspondingto the same high material level. A still higher capacitance differentialmay be established with jumper 84 removed and jumper 86 intact. Formaterials of relatively high dielectric constant such as glycerine, bothjumpers 84, 86 may remain intact, so that resistors 80, 82, 87 inparallel establish a maximum capacitance differential of twentypicofarads, for example.

When the output from phase detector 32 decreases from the empty tanknear-resonance point established as a result of the automaticcalibration operation previously described to a level established byresistors 56, 62, 80, 82 and/or 87, the output of differentialcomparator 70 switches from a low or logical zero level to a high orlogical one level, thereby indicating proximity of material to the probeassembly. A resistor 88 is connected between the output of comparator 70and the non-inverting input thereof to establish a hysteresis incomparator operation and thereby avoid intermittent switching ofcomparator output at a borderline material level condition. The outputof differential comparator 70 is also connected in material levelindicating circuitry 38 to one input of an exclusive-or gate 90. Thesecond input of gate 90 is connected through a jumper 92 to the positivevoltage supply and through a resistor 92 to ground. The output of gate90 is connected through a resistor 95 to the base of an NPN transistor96 which functions as an electronic switch to illuminate an LED 98through a resistor 99 and to energize a relay coil 100 when the outputof gate 90 assumes a high or logical one condition. The contacts 102associated with relay coil 100 are connected to corresponding terminalsof a terminal block 104 for connection to external circuitry aspreviously described.

Jumper 92 and resistor 94 cooperate with gate 90 for selecting eitherlow level or high level fail safe operation of material level indicatingcircuitry 38. That is, jumper 92 and resistor 94 cooperate with gate 90to deenergize relay coil 100 at either a high level condition (materialproximate to probe 20) or a low level condition (material spaced fromprobe 20). In this way, the selected high level or low level conditionwill also be indicated to the external circuitry independently of actualmaterial level in the event that relay coil 100 is de-energized by apower failure or the like. As previously indicated, the output ofcomparator 70 assumes a high or logical one voltage level when material28 is in proximity to probe 20 (FIG. 1). If low level fail safeoperation is desired, which means that relay 100 will de-energize toindicate a low material level, jumper 92 is removed so as to place a lowor logical zero at the second input of gate 90. In this configuration,the output of gate 90 follows the first input from comparator 70 toilluminate LED 98 and energize relay coil 100 whenever material is inproximity to the capacitance probe, and to de-energize the LED and relaycoil when material is spaced from the probe (low level). On the otherhand, in high level fail safe operation with jumper 92 left intact, ahigh or logical one voltage level is placed at the second input of gate90, so that the gate output is the inverse of the first input fromcomparator 70. LED 98 is thus illuminated and relay coil 100 energizedwhen the level of material is remote from the sensor probe (low level)and de-energized when the material is in proximity to the probe (highlevel).

FIGS. 4 and 5 illustrate respective modifications to the preferredembodiment of the invention hereinabove described. Only the differencesbetween the respective modifications and the preferred embodiment areillustrated in FIGS. 4 and 5 and described hereinafter. In themodification of FIG. 4, the adjustable LC resonant circuit of thepreferred embodiment is replaced by a non-adjustable resonant circuitcomprising a fixed capacitor 104 and a fixed inductor 102 connected inparallel with each other and with probe 20. Most preferably, capacitor104 and inductor 102 are selected so as to exhibit resonance at thefrequency of oscillator 10 in combination with an empty-tank capacitanceat probe 20 (uncoated), such as fifteen picofarads for example. Thebit-parallel data output of counter 50 is fed to a digital-to-analogconverter 16 which provides an analog output voltage to referenceresistor 56 in place of the fixed power supply voltage in the preferredembodiment of the invention previously described so as to vary thereference input voltage to comparator 54 as a function of the count incounter 50. Differential comparator 54 receives inputs from phasedetector 30 and reference resistor 56, and terminates operation of theautomatic calibration mode by removing the enabling input from counter50 when the reference voltage supplied by resistor 56 to the invertinginput of comparator 54 is equal to the output from phase detector 32corresponding to the coated probe empty vessel calibration condition atprobe 20. The corresponding count in counter 50 is held and the outputof d/a converter 106 thereafter remains constant so as to provide anempty-tank reference voltage to threshold detector 36 as previouslydescribed. Thus, the reference level supplied by resistors 56, 62 tothreshold detector 36 (FIGS. 1 and 3) in the modified calibrationcircuit of FIG. 4 indicates both empty-tank probe capacitance and anyaddition thereto caused by material coated on the probe and noteffectively blocked by guard shield 26.

FIG. 5 illustrates a modified adjustable resonant circuit 107 whichincludes a source 108 of direct current controlled by the output of d/aconverter 106 and connected to coil 102 for varying the effectiveinductance thereof as a function of the count in counter 50. Currentsource 108 includes a PNP transistor 110 having an emitter connectedthrough a capacitor 112 to the output of amplifier 12 (FIG. 1) andthrough a resistor 114 to a positive d.c. voltage source. The base oftransistor 110 is connected to the output of d/a converter 106 andthrough a resistor 116 to the voltage source. The collector oftransistor 110 is connected to the parallel resonant circuit comprisinginductor 102, capacitor 104 and probe 20. As the count in counter 50increases during the automatic calibration mode, the direct current fedby transistor 110 to coil 102 decreases correspondingly, and therebydecreases the effective a.c inductance of coil 102 to compensate forincreased probe capacitance caused by material coating. When theeffective inductance is decreased to a point where the parallelcombination of capacitance 104, inductor 102 and coated probe 20 aresubstantially at resonance, operation of counter 50 is terminated bydifferential comparator 54 (FIG. 2) in the manner previously described.Resistor 116 may be variable for factory adjustment of current sourcegain.

FIG. 6 illustrates a presently preferred embodiment of probe assembly 20in accordance with the invention as comprising a closed housing 120 inwhich all of the system electronics hereinabove are preferably disposed.Housing 120 includes a conductor opening 122 for receiving power from asource remote from the probe assembly, for receiving a calibrationsignal from switch 40 (FIGS. 1 and 2) which may also be located remotelyof the probe assembly, and for connection of terminal block 104 (FIG. 3)to external circuitry. An externally threaded nipple 124 projectsintegrally from one wall of housing 120. Telescopically received withinnipple 124 is a probe sub-assembly comprising probe conductor 18 in theform of a solid metal rod, a hollow tubular guard shield 26 receivedover rod 18 and disposed intermediate the ends of rod 18, and insulatingmaterial 126 surrounding rod 18 and separating rod 18 from shield 26.The ends of probe rod 18 and shield 26 remote from nipple 124 areexposed--i.e., not covered by insulating material. An externallythreaded adapter 130 is received within nipple 124 and sealinglycaptures therein a conical shoulder 128 integrally formed of insulatingmaterial 126.

The exposed portion of adapter 130 is externally threaded so as to beadapted to be received within an internally threaded gland disposed atdesired position on the material vessel so that probe rod 18 and guardshield 26 project internally of the vessel as shown schematically inFIG. 1. As shown in FIG. 6, guard shield 126 terminates withininsulating material 126 at a position spaced from shoulder 128 and hassoldered or otherwise connected thereto an insulated conductor 132.Conductor 132 is spirally wound around rod 18 or otherwise physicallyattached thereto, and extends into housing 120 for connection toamplifier 24 (FIG. 1). Likewise, a conductor 134 is fastened interiorlyof housing 122 to probe rod 18 for connection to resonant circuit 14(FIG. 1). Preferably, rod 18, guard shield 26, insulating material 126and conductor 132 are formed as a sub-assembly by fixturing guard shield26 with respect to rod 18 and then injection molding insulating material126 around the guard shield and probe rod. An insulating washer 136 maybe positioned between rod 18 and guard shield 26 to facilitate suchfixturing.

The several embodiments of the invention exhibit a number of significantadvantages over prior art devices of similar type. For example,responsiveness of the material level detection circuitry to phase angleof the probe signal rather than to amplitude thereof renders the leveldetection circuitry substantially unresponsive to variations inconductivity of the material caused by varying moisture content, etc.All embodiments may be readily recalibrated at any time by an operatorby merely depressing switch 40 (FIGS. 1 and 2) and holding the switchdepressed until LED 60 (FIG. 2) illuminates. The indicators may bemanufactured to be identical at the factory and modified to suitparticular applications in the field by removing one or more jumpers 76,78, 84, 86 according to prespecified factory directions. This featurereduces the number of models which must be stocked by a distributor.

It will be appreciated that the invention is susceptible to a number ofmodifications and variations in addition to those hereinabove describedin detail. For example, although automatic calibration in accordancewith the embodiments of FIGS. 2, 4 and 5 is presently preferred, manualcalibration may be provided in accordance with the present invention inits broadest aspects by providing a manually adjustable capacitanceand/or inductance in the parallel resonance circuit, or by providing formanual adjustment of the reference resistor 56 in a manner analogous tothe automatic adjustment embodiment of FIG. 4.

The broad concepts or principles embodied in FIGS. 1 and 3, as well asthe specific auto-calibration circuits of FIGS. 2 and 5, are the subjectof concurrently filed application Ser. No. 411,527 assigned to theassignee hereof. The method of constructing probe assembly so describedin connection with FIG. 6 is the subject of concurrently filedapplication Ser. No. 411,524 assigned to the assignee hereof.

The invention claimed is:
 1. A system for indicating the level ofmaterial in a vessel as a function of material capacitance comprising aresonant circuit including a capacitance probe adapted to be disposed ina vessel so as to be responsive to variations in capacitance as afunction of material level in said vessel, an oscillator having anoutput coupled to said resonant circuit including said capacitanceprobe, phase detection means responsive to variations in phase angle ofsaid oscillator output as a function of probe capacitance, calibrationmeans for identifying a reference capacitance indicative of apredetermined level of material in said vessel, and means responsive tosaid phase detection means and to said calibration means for indicatingmaterial level in said vessel as a function of a difference betweencapacitance at said probe and said reference capacitance, saidcalibration means comprising comparison means having a first inputresponsive to said phase detection means, a second input and an outputresponsive to a comparison between said first and second inputs,reference means connected to said second input and indicative of saidreference capacitance, means for initiating a calibration operation,counter means including means responsive to said initiating means forresetting said counter means, means responsive to said comparison meansfor enabling operation of said counter means during said calibrationoperation, and means for varying the count in said counter means duringsaid calibration operation, means responsive to the count in saidcounter means for varying said reference means connected to said secondinput of said comparison means as a function of said count, and meansresponsive to said output of said comparison means for terminating saidcalibration operation when said output indicates a predeterminedcomparison between said first input from said phase detection means andsaid second input from said reference means.
 2. The system set forth inclaim 1 wherein said comparison means comprises means responsive to adifference in voltage between said first input from said phase detectionmeans and said second input from said reference means, and wherein saidcount responsive means comprises means for varying the reference voltageapplied to said second input by said reference means.
 3. The system setforth in claim 2 wherein said counter means includes a digital counter,and wherein said count-responsive means includes digital-to-analogconverter means for applying an analog voltage to said second input as afunction of said count.
 4. The system set forth in claims 1, 2 or 3wherein the count-responsive means varies said reference means to obtainsaid predetermined comparison substantially at resonance of saidresonant circuit.
 5. A system for indicating the level of material in avessel as a function of material capacitance comprising a resonantcircuit including a capacitance probe adapted to be disposed in a vesselso as to be responsive to variations in capacitance as a function ofmaterial level in said vessel, an oscillator having an output coupled tosaid resonant circuit including said capacitance probe, phase detectionmeans responsive to variations in phase angle at said oscillator outputas a function of probe capacitance, calibration means for identifying areference capacitance indicative of a predetermined level of material insaid vessel, and means responsive to said phase detection means and tosaid calibration means for indicating material level in said vessel as afunction of a difference between capacitance at said probe and saidreference capacitance, said calibration means comprising comparisonmeans having a first input responsive to said phase detection means, asecond input and an output responsive to a comparison between said firstand second inputs, reference means connected to said second input andindicative of said reference capacitance, means for initiating acalibration operation, means responsive to said initiating means forproviding a varying d.c. signal, means responsive to said d.c. signalfor varying said reference means connected to said second input of saidcomparison means as a function of said d.c. signal, and means responsiveto said comparison means for terminating variation of said d.c. signalat a level indicative of a predetermined comparison between said firstinput from said phase detection means and said second input from saidreference means.